The invention described herein may be manufactured and used by and for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefor.
Not applicable.
1. Field of the Invention
The invention relates to a sparse imaging array and is directed more particularly to such an array for underwater use and which requires fewer transducer elements and provides a wider area of focusing than prior art arrays.
2. Description of the Prior Art
Two dimensional arrays of underwater acoustic transducers are known. Such arrays are made by providing relatively large monolithic plates of piezo-ceramic transducer material. The plates are then cut along a series of parallel lines extending in a selected direction, and then cut along a series of parallel lines normal to the aforementioned lines, to provide a multitude of small square or rectangular block elements. A selected viscoelastic material is packed into the cut-away areas to decouple the block elements from each other.
Selected ones of the block transducer elements are then wired for operation. The remaining elements provide no benefit. Typically, only about 1%, or less, of the transducer elements are selected for wiring. In a known array, about 250,000 block elements are produced by the above-described technique, known as xe2x80x9cdice and fillxe2x80x9d. About 1700 of the formed elements are then wired to become active elements. There is a need for an array in which such waste of materials is avoided and related costs are reduced.
Further, it is beneficial to place the active elements in positions selected with precision. However, given that the active elements of the above-described known array necessarily reside in areas defined by criss-crossing lines, the active element which is closest to the desired location is used in practice. In short, the active elements are located approximately where wanted, but not usually precisely where wanted. There is a need for an array in which the active elements are placed precisely where wanted.
Still further, the spatial response of each rectangular block element is perturbed by non-resonant lateral waves traveling in the plane of the array. Such waves occur at a critical angle based on the relative sound speeds of the array material and the surrounding fluid, typically sea water. Passing through the piezo ceramic element, such lateral waves cause an out of phase voltage with respect to a desired mode voltage and essentially limit the element beam width. There is a need for an array with improved element beamwidth.
An object of the invention is, therefore, to provide an ultrasonic sparse imaging array for underwater use, the array including a selected number of active transducer elements with no inactive transducer elements.
A further object of the invention is to provide such an array in which each transducer element is located precisely where desired on a substrate.
A still further object of the invention is to provide such an array having elements which provide a wider beam width to provide the array with a larger field of view.
A still further object of the invention is to provide a low cost method for making an ultrasonic sparse imaging array having the attributes noted immediately above.
With the above and other objects in view, a feature of the present invention is the provision of an ultrasonic sparse imaging array comprising a backing substrate of highly acoustically absorptive material, a multiplicity of holes extending through the substrate, a multiplicity of adhesive sheets having selectively conductive regions, the sheets each being fixed to a first side of the backing substrate and disposed over a first end of one of the holes, to provide a mechanical and electrical connection between the substrate and a multiplicity of transducer elements within the array. Plano-convex shaped transducer elements, each having a wide acoustic beamwidth, are respectively disposed on each of the sheets. Each of the sheets constitutes a positive electrode. A plating is fixed to the first side of the substrate, covering each of the transducer elements and constitutes a negative electrode. A conductive pin is disposed in each of the holes, the pins each being provided with an annular disc portion which closes second ends of the holes. A conductive epoxy fills each of the holes between the pin disc and the sheet. A power source is provided and is in electrical communication with the plating.
In accordance with a further feature of the invention, there is provided a method for making an ultrasonic sparse imaging array, the method comprising the steps of providing a substrate of highly absorptive material, drilling a multiplicity of holes through the substrate in a selected pattern, injecting conductive epoxy into the holes, inserting conductive pins, one each, into the holes, the pins each having an annular disc portion which is brought into engagement with the substrate undersurface to close off undersurface ends of the holes, removing epoxy overflowed from the holes from an undersurface and an upper surface of the substrate, fixing a sheet of dry film adhesive with selectively conductive regions, and comprising a positive electrode, on the upper surface of the substrate, and fixing a generally plano-convex shaped transducer element on each of the sheets, disposing a plating on the upper surface of the substrate, the plating covering the transducer elements and comprising a negative electrode, and providing connections on the plating for placing the plating in electrical communication with a power source.